The invention relates to a method and an apparatus for determining spatial coordinates at a multiplicity of measurement points along a contour of a measurement object. In particular, the invention relates to a method and an apparatus for the continuous measurement (so-called scanning) of a surface of a measurement object by means of an active tactile probe head that enables the setting of a defined contact force (or sensing force) by means of a preliminary deflection.
The present invention is applied in the field of so-called coordinate measuring technology. This is understood as measuring workpieces in one, two and preferably even three dimensions by means of measuring tools that enable spatial coordinates to be determined at selected measurement points of the workpieces. A typical coordinate measuring machine has a head part that can be moved relative to the workpiece or measurement object. The head part carries a sensor by means of which the head part can be brought into an exactly defined position with reference to the measurement point. In tactile coordinate measuring machines, the measurement point is contacted by means of a probe element. The head part is therefore usually denoted as probe head in such cases. The probe element is generally a stylus whose free end terminates in a spherical tip. The stylus is movably supported on the probe head such that the stylus is deflected relative to the probe head during contacting. Given a specific deflection and/or contact force, position measuring values are recorded that are representative of the position of the probe head in the measuring volume, and thus of the position of the probe head relative to the measurement object. Moreover, in the case of so-called measuring probe heads the deflection of the stylus relative to the probe head is determined. Spatial coordinates for the contacted measurement point can then be determined from the position measuring values for the probe head and the deflections of the stylus. By contacting a plurality of surface points on a workpiece, it is also possible to detect geometric dimensions and shape profiles, the latter being possible with particular effectiveness by using a continuous (scanning) recording of measurement values.
In the case of probe heads for tactile coordinate measuring machines, one can distinguish between active and passive probe heads. In the case of passive probe heads, the rest position of the probe element is set solely by means of mechanical springs. A deflection from the rest position takes place only during contacting of a measurement point against the spring force. By contrast, active probe heads have one or more actuators configured to deflect the probe element in a defined way before contact with the measurement object takes place, or without contact taking place. Such active probe heads are particularly suitable for scanning measurements, because the actuator can be used for keeping the probe element in continuous contact with the surface of the measurement object.
However, the invention is not limited to coordinate measuring machines in the narrower sense. It can, for example, also be used on machine tools or other machines in the case of which a workpiece surface is scanned with a tactile probe element.
Even in the case of active probe heads, the probe element is supported on the probe head by means of spring elements in order to enable the deflection about the rest position. In the case of active and passive probe heads, the support by means of spring elements renders it possible for the probe element to get into mechanical oscillations because of the movements of the probe head and because of the contacting actions.